Often 2 variants of feminine instrumental...?

Question

maybe my title needs some work... but I use Wiktionary a lot for declensions, etc and I often see two forms for feminine instrumental adjectives, but my textbook only ever uses, typically, the 'first' one...

For example, https://en.m.wiktionary.org/wiki/%D0%BA%D0%BE%D1%82%D0%BE%D1%80%D1%8B%D0%B9 for который, or for какой.

который -> fem. instr. -> кото́рой, кото́рою
какой -> fem. instr. -> како́й, како́ю

The same problem for me exists with singular instrumental for nouns like книга - https://en.m.wiktionary.org/wiki/%D0%BA%D0%BD%D0%B8%D0%B3%D0%B0#Russian

какой ->  sing. instr. -> кни́гой, кни́гою

Why are there two forms in these and many other words? What is the reason? When is each used?

Answer 1

The -ою/-ею forms are older and sound archaic/poetic. -ой/-ей is the modern form, reflecting the same trend towards loss of semantically weak final vowels that turned -ти infinitives into -ть.

With the exception of ею, the instrumental of the pronoun она which some prescribe as the preferred literary norm since ей is easily confused with the dative form, no -ою/-ею form is ever mandatory or preferred; in fact, unless there's a clear intent at mimicking an older literary style, these forms should probably be avoided. And even in actual older texts, you'll likely find them in free variation with -ой/-ей.

Answer 2

The usual ending of feminine adjectives in the instrumental is -ой or -ей.

Летом мы лакомились вкусной ягодой. Вода была горячей-прегорячей.

Feminine adjectives can have the ending -ою,-ею in the instrumental case.(вариантные окончания). They are usef mainly in poetry, folklore or bookish style.

Берёзы жёлтою резьбой

Блестят в лазури голубой.

И. Бунин.

Под сосною зеленою спать положите вы меня.( Песня «Калинка»).

Sometimes the noun also uses the same poetic ending.